The development of the immune system and the generation of specific immune responses are regulated by a network of structurally related growth and differentiation factors, such as the prototypical interleukin-2 (IL-2). Like other cytokines, IL-2 exerts its effects through a multimeric transmembrane receptor complex displayed on the surface of a variety of lymphocytes. The gamma chain (IL-2R-gamma, gamma-c) of this receptor undoubtedly is integral to the transmembrane signaling function of this complex, since natural deletions affecting its cytoplasmic tail severely compromise lymphocyte function in the devastating human disease X-linked severe combined immunodeficiency (X-SCID). Moreover, this chain is a key component of receptors for several other interleukins including IL-4, IL- 7, IL-9 and IL-15. Little is known about gamma-c substructures, receptor- associated signaling intermediates, and downstream effectors that together mediate the common and distinctive signal transduction functions of the receptor systems that share this subunit. Furthermore, the specific roles of gamma-c in lymphoid development, normal immune system functioning, and leukemogenesis in vivo are poorly characterized. The proposed studies will use biochemical, cellular and gene transfer methods to define the molecular basis and consequences of transmembrane signaling involving the gamma-c receptor subunit. In the first stage a panel of site-directed mutants of the gamma-c intracellular segment will be characterized using chimeric receptor systems and assay methods established in preliminary studies. By these studies the functional architecture of the gamma-c subunit will be defined as it relates to early and late signaling events regulated by the IL-2, IL-4, IL-7 and IL-9 receptors. Both shared and distinctive functional domains of the gamma-c chain will thus be elucidated. Second, biochemical, cell biologic and molecular methods will be employed to identify intracellular signaling intermediates associated with the cytoplasmic tail of gamma-c. Structure/function correlations with this signal transduction complex will be made with the data derived from the mutagenesis and biochemical studies. Third, hypotheses derived from these in vitro approaches will be tested using a recently developed gene deletion animal model of X-SCID. Such a system will allow a detailed characterization of the in vivo role played by gamma-c in lymphoid maturation and in the generation of a coordinated immune response. Collectively, the studies performed under the specific aims of this proposal will define the molecular basis of signal transduction by gamma- c-containing cytokine receptors, and of the specific contributions of the gamma-c chain to these processes. Furthermore, these investigations will elucidate the significance of these signaling events in mammalian immune function(s). Such information will serve as a basis for understanding pathologic processes involving hypo- or hyperactive signal transduction by such receptors, including X-SCID and possibly leukemogenesis, as well as for designing rational therapeutic strategies.